This document provides definitions for various biology terms. It indicates that terms in red are IB action verbs and terms in blue are IB syllabus-required definitions. The definitions provided are intended as aids to recall rather than formal definitions. Key terms defined include abiotic factor, absorption spectrum, active transport, aerobic respiration, allele, antigen, ATP, cell, chromosome, DNA, enzyme, gene, homeostasis, and photosynthesis among others.

1. ■ Entries that are IB action verbs
and entries that are IB syllabus-
required definitions are coloured
red and blue respectively.
■ Entries are aides-mémoire, rather
than formal definitions.
A
abiotic factor a non-biological factor
(e.g. temperature) that is part of the
environment of an organism
abscisic acid a plant growth substance
tending to inhibit growth
absorption spectrum range of a
pigment’s ability to absorb various
wavelengths of light
acetylcholine a neurotransmitter,
liberated at synapses in the CNS
acid rain the cocktail of chemical
pollutants that may occur in the
atmosphere
action potential rapid change
(depolarisation) in membrane
potential of an excitable cell (e.g. a
neurone)
action spectrum range of wavelengths
of light within which a process like
photosynthesis takes place
activation energy energy a substrate
molecule must have before it can
undergo a chemical change
active site region of enzyme molecule
where substrate molecule binds
active transport movement of
substances across a membrane
involving a carrier protein and
energy from respiration
adenine a purine organic base, found
in the coenzymes ATP and NADP,
and in nucleic acids (DNA and
RNA) in which it pairs with thymine
adenosine diphosphate (ADP) a
nucleotide, present in every living
cell, made of adenosine and two
phosphate groups linked in series,
and important in energy transfer
reactions of metabolism
adenosine triphosphate (ATP) a
nucleotide, present in every living
cell, formed in photosynthesis and
respiration from ADP and Pi, and
functioning in metabolism as a
common intermediate between
energy-requiring and energy-
yielding reactions
adrenaline a hormone secreted by the
adrenal medulla (and a
neurotransmitter secreted by nerve
endings of the sympathetic nervous
system), having many effects,
including speeding of heart beat,
and the breakdown of glycogen to
glucose in muscle and liver
aerobic respiration respiration
requiring oxygen, involving
oxidation of glucose to carbon
dioxide and water
alimentary canal the gut; a tube
running from mouth to anus in
vertebrates, where complex food
substances are digested and the
products of digestion selectively
absorbed into the body
allele an alternative form of a gene,
occupying a specific locus on a
chromosome
allele frequency the commonness of
the occurrence of any particular
allele in a population
alpha cell (pancreas) glucagon-
secreting cell of the islets of
Langerhans in the pancreas
alveolus air sac in the lung
amino acid building block of proteins,
of general formula
R.CH(NH2).COOH
anabolism the building up of complex
molecules from smaller ones
anaerobic respiration respiration in
the absence of oxygen, involving
breakdown of glucose to lactic acid
or ethanol
analogous structure similar in
structure but of different
evolutionary origin
analyse interpret data to reach a
conclusion
anion negatively charged ion
annotate add brief notes to a diagram,
drawing or graph
anther part of the stamen in flowers,
consisting of pollen sacs enclosed in
walls that eventually split open,
releasing pollen
antibody a protein produced by blood
plasma cells derived from B
lymphocytes when in the presence
of a specific antigen, which then
binds with the antigen, aiding its
destruction
antibiotics organic compounds
produced by some microorganisms
which selectively inhibit or kill
other microorganisms
anticodon three consecutive bases in
tRNA, complementary to a codon
on RNA
antidiuretic hormone (ADH)
hormone secreted by the pituitary
gland that controls the permeability
of the walls of the collecting ducts
of the kidney
antigen a substance capable of
binding specifically to an antibody
apoplast collective name for the cell
walls of a tissue or plant
apply use an idea, equation, principle,
theory, or law in a new situation
aqueous humour fluid between lens
and cornea of the eye
arteriole a very small artery
artificial classification classifying
organisms on the basis of few, self-
evident features
artificial selection selection in
breeding exercises, carried out
deliberately, by humans
asexual reproduction reproduction not
involving gametes and fertilisation
assimilation uptake of nutrients into
cells and tissues
atherosclerosis deposition of plaque
(cholesterol derivative) on inner
wall of blood vessels
atrio-ventricular node mass of tissue
in the wall of the right atrium,
functionally part of the pacemaker
mechanism
atrio-ventricular valve tricuspid or
bicuspid valve
atrium (plural, atria) one of the two
upper chambers of the mammalian
four-chambered heart
Glossary

2. B408 Glossary
autolysis self-digestion
autotrophic (organism) self-feeding –
able to make its own elaborated
foods from simpler substances
autonomic the involuntary nervous
system
auxin plant growth substance,
indoleacetic acid
axon fibre carrying impulses away
from the cell body of a neurone
B
bacillus a rod-shaped bacterium
bacteriophage a virus that parasitises
bacteria (also known as a phage)
baroreceptor a sensory receptor
responding to stretch, in the walls
of blood vessels
basement membrane the thin fibrous
layer separating an epithelium from
underlying tissues
beta cell (pancreas) insulin-secreting
cells of the islets of Langerhans in
the pancreas
bicuspid valve valve between atrium
and ventricle on the left side of the
mammalian heart
bile an alkaline secretion of liver cells
which collects in the gall bladder in
humans, and which is discharged
into the duodenum periodically
binary fission when a cell divides
into two daughter cells, typically in
reproduction of prokaryotes
binomial system double names for
organisms, in Latin, the generic
preceding the specific name
biological pest control control of
pests and weeds by other organisms
biomagnification the process by which
chemical substances become more
concentrated at each trophic level
biomass total mass of living organisms
in a given area (e.g. a quadrat)
biome a major life-zone over an area
of the Earth, characterised by the
dominant plant life present
biosphere the inhabited part of the
Earth
biotechnology the industrial and
commercial applications of biology,
particularly of microorganisms,
enzymology and genetic engineering
biotic factor the influence of living
things on the environment of other
living things
bivalent a pair of duplicated
chromosomes, held together by
chiasmata during meiosis
blastocyst embryo as hollow ball of
cells, at the stage of implantation
blind spot region of the retina where
the optic nerve leaves
body mass index (BMI) body mass in
kg/(height in m)2
bone marrow tissue special
connective tissue filling the cavity
of certain bones
boreal forest northern coniferous
forests (example of a biome)
bovine somatotrophine (BST)
hormone produced by the pituitary,
controlling milk production
brain the coordinating centre of the
nervous system
breed (animal) the animal equivalent
of a plant variety
bronchiole small terminal branch of a
bronchus
bronchus a tube connecting the
trachea with the lungs
brush border tiny, finger-like
projections (microvilli) on the
surface of epithelial cells of the
small intestine
buffer a solution which minimises
change in pH when acid or alkali
are added
bundle of His bundles of long muscle
fibres that transmit myogenic
excitation throughout the ventricle
walls
C
C3 pathway the light-independent
reaction in photosynthesis,
producing as its first product, a
3-carbon compound, glycerate
3-phosphate
C4 plants plants with an additional
carbon dioxide-fixation pathway
that augments the supply of this raw
material of photosynthesis at the
chloroplast
calculate find an answer using
mathematical methods
Calvin cycle a cycle of reactions in
the stroma of the chloroplast by
which some of the product of the
dark reaction is re-formed as the
acceptor molecule for carbon
dioxide (ribulose biphosphate)
carrier an individual that has one
copy of a recessive allele that causes
a genetic disease in individuals that
are homozygous for this allele
carrier protein one of the types of
protein in plasma membranes,
responsible for active transport
across the membranes
cardiac cycle the stages of the heart
beat, by which the atrial and then
the ventricle walls alternately
contract (systole) and relax
(diastole)
carnivore flesh-eating animal
cartilage firm but plastic skeletal
material (e.g. cartilage over bones
at joints)
Casparian strip band of cells with
impervious walls, found in plant
roots
catabolism the breaking down of
complex molecules in the
biochemistry of cells
catalyst a substance that alters the
rate of a chemical reaction, but
remains unchanged at the end
cellular respiration controlled release
(transfer) of energy from organic
compounds in cells to form ATP
cellulase enzyme capable of
hydrolysing cellulose
cellulose an unbranched polymer of
2000–3000 glucose residues, the
major ingredient of most plant walls
central dogma the idea that transfer
of genetic information from DNA
of the chromosome to mRNA to
protein (amino acid sequence) is
irreversible
centromere constriction of the
chromosome, the region that
becomes attached to the spindle
fibres in division
centrosome organelle situated near
the nucleus in animal cells,
involved in the formation of the
spindle prior to nuclear division
cephalisation development of a head
at the anterior of an animal
cerebellum part of hindbrain,
concerned with muscle tone,
posture and movement
cerebral cortex superficial layer of grey
matter on extension of forebrain,
much enlarged in humans and apes

3. Glossary B409
cerebral hemispheres (cerebrum) the
bulk of the human brain, formed
during development by the
outgrowth of part of the forebrain,
consisting of densely packed
neurones and myelinated nerve
fibres
chemoautotroph an organism that
uses energy from chemical reactions
to generate ATP and produce
organic compounds from inorganic
substances
chemoheterotroph an organism that
uses energy from chemical reactions
to generate ATP and obtains
organic compounds from other
organisms
chemoreceptor a sense organ
receiving chemical stimuli
chemosynthesis use of chemical
energy from oxidation of inorganic
compounds to synthesise organic
compounds, typically from carbon
dioxide and water
chiasma (plural, chiasmata) site of
crossing over (exchange) of
segments of DNA between
homologous chromosomes
chloroplast organelle that is site of
photosynthesis and contains
chlorophyll
chlorophyll the main photosynthetic
pigment of green plants, occurs in
the grana membranes (thylakoid
membranes) of the chloroplasts
cholesterol a lipid of animal plasma
membranes; a precursor of the
steroid hormones, in humans,
formed in the liver and transported
in the blood as lipoprotein
chromatid one of two copies of a
chromosome after it has replicated
chromatin a nuclear protein material
in the nucleus of eukaryotic cells at
interphase; forms into chromosomes
during mitosis and meiosis
choroid layer of blood vessels lying
below the retina
chromosome visible in appropriately
stained cells at nuclear division,
each chromosome consists of a long
thread of DNA packaged with
protein; chromosomes replicate
prior to division, into chromatids.
Contents of nucleus appears as
granular chromatin between
divisions
chyme partly digested food as it
leaves the stomach
cilium (plural, cilia) motile, hair-like
outgrowth from surface of certain
eukaryotic cells
citric acid cycle see Krebs cycle
clade the branch of a phylogenetic
tree containing the set of all
organisms descended from a
particular common ancestor which
is not an ancestor of any non-
member of the group
cladistics method of classifying living
organisms that makes use of lines of
descent only (rather than
phenotypic similarities)
climax community the mature
(stable) stage of a succession of
communities
clone a group of genetically identical
individuals (or cells)
coccus spherical bacterial cell
CNS see nervous system
codominant alleles pairs of alleles
that both affect the phenotype
when present in a heterozygous
state
codon three consecutive bases in
DNA (or RNA) which specify an
amino acid
coleoptile protective sheath around
emerging leaves of germinating
grass seeds
colon part of the gut, preceding the
rectum
colostrum first milk secreted by the
mother, after birth of young
commensalism a mutually beneficial
association between two organism
of different species
comment give a judgement based on
a given statement or result of a
calculation
community a group of populations of
organisms living and interacting
with each other in a habitat
compare give an account of
similarities and differences between
two or more items (e.g. by using a
table)
compensation point the point where
respiration and photosynthesis are
balanced
condensation reaction formation of
larger molecules involving the
removal of water from smaller
component molecules
cone (retinal cell) a light-sensitive
cell in the retina, responsible for
colour vision
conjugate protein protein combined
with a non-protein part
connective tissue tissues that support
and bind tissues together
conservation applying the principles of
ecology to manage the environment
contractile vacuole a small vesicle in
the cytoplasm of many fresh water
protozoa that expels excess water
construct represent or develop in
graphic form
cornea transparent covering at the
front of the eye
corpus luteum glandular mass that
develops from an ovarian follicle in
mammals, after the ovum is
discharged
cotyledon the first leaf (leaves) of a
seed plant, found in the embryo
covalent bond bond between atoms
in which electrons are shared
cristae folds in the inner membrane
of mitochondria
crossing over exchange of genetic
material between homologous
chromosomes during meiosis
crypt of Lieberkuhn endocrine cells
within the pancreas
cuticle layer of waxy material on
outer wall of epidermis
cyanobacteria photosynthetic
prokaryotes
cytokinesis division of cytoplasm
after nucleus has divided into two
cytology study of cell structure
cytoplasm living part of the cell
bound by the plasma membrane,
excluding the nucleus
cytosol what remains of cytoplasm
when the organelles have been
removed
D
data recorded products of
observations and measurements
qualitative data observations not
involving measurements
quantitative data precise observat-
ions involving measurements

4. deamination the removal of NH2
from an amino acid
deciduous loss at the end of the
growing season (e.g. of leaves from
broadleaved trees)
decomposer organisms (typically
microorganisms) that feed on dead
plant and animal material, causing
matter to be recycled by other
living things
degenerate code the triplet code
contains more codons than there
are amino acids to be coded, so
most amino acids are coded for by
more than one codon
deduce reach a conclusion from the
information given
define give the precise meaning of a
word or phrase as concisely as
possible
denaturation a structural change in a
protein that results in a loss (usually
permanent) of its biological
properties
dendrite a fine fibrous process on a
neurone that receives impulses from
other neurones
depolarisation (of axon) a temporary
and local reversal of the resting
potential difference of the
membrane that occurs when an
impulse is transmitted along the
axon
derive manipulate a mathematical
equation to give a new equation or
result
describe give a detailed account
including all relevant information
desertification the conversion of
marginal cultivated land into desert,
caused by climate change or by
over-grazing or inferior cultivation
design produce a plan, object,
simulation or model
determine find the only possible
answer
detrital chain a food chain based on
dead plant matter
detritivore an organism that feeds on
detritus (dead organic matter)
dialysis separation of large and small
molecules in solution by the
inability of the former to pass
through a selectively permeable
membrane
diaphragm a sheet of tissues, largely
muscle, separating thorax from
abdomen in mammals
diastole relaxation phase in the
cardiac cycle
dichotomous key one in which a
group of organisms is progressively
divided into two groups of smaller
size
dicotyledon class of
Angiospermophyta having an
embryo with two seed leaves
(cotyledons)
diffusion passive movement of
particles from a region of high
concentration to a region of low
concentration
dihybrid cross one in which the
inheritance of two pairs of
contrasting characters (controlled
by genes on separate chromosomes)
is observed
diploid condition organisms whose
cells have nuclei containing two
sets of chromosomes
disaccharide a sugar that is a
condensation product of two
monosaccharides (e.g. maltose)
discuss give an account including,
where possible, a range of
arguments, assessments of the
relative importance of various
factors or comparisons of alternative
hypotheses
distinguish give a difference between
two or more different items
disulphide bond S—S bond between
two S-containing amino acid
residues in a polypeptide or protein
chain
diuresis increased secretion of urine
division of labour the carrying out of
specialised functions by different
types of cell in a multicellular
organism
DNA a form of nucleic acid found in
the nucleus, consisting of two
complementary chains of
deoxyribonucleotide subunits, and
containing the bases adenine,
thymine, guanine and cytosine
dominant allele an allele that has the
same effect on the phenotype
whether it is present in the
homozygous or heterozygous state
double bond a covalent bond
involving the sharing of two pairs of
electrons (rather than one)
double circulation in which the
blood passes twice through the
heart (pulmonary circulation, then
systemic circulation) in any one
complete circuit of the body
double fertilisation a feature of
flowering plants in which two male
nuclei enter the embryo sac, and
one fuses with the egg cell and one
with the endosperm nucleus
draw represent by means of pencil
lines (with labels added)
duodenum the first part of the
intestine after the stomach
E
ecology the study of relationships
between living organisms and
between organisms and their
environment – a community and its
abiotic environment
ecosystem a natural unit of living
(biotic) components and non-living
(abiotic) components (e.g.
temperate deciduous forest)
edaphic factor factor influenced by
the soil
effector an organ or cell that
responds to a stimulus by doing
something (e.g. a muscle
contracting, a gland secreting)
egestion disposal of waste from the
body (e.g. defecation)
egg cell an alternative name for an ovum
electron microscope (EM) microscope
in which a beam of electrons
replaces light, and the powers of
magnification and resolution are
correspondingly much greater
electron-transport system carriers
that transfer electrons along a redox
chain, permitting ATP to be
synthesised in the process
embolism a blood clot blocking a
blood vessel
embryo the earliest stages in
development of a new animal or
plant, from a fertilised ovum,
entirely dependent on nutrients
supplied by the parent
embryo sac occurs in the ovule of
flowering plants, and contains the
egg cell and endosperm nucleus
B410 Glossary

5. Glossary B411
emulsify to break fats and oils into
very tiny droplets
endemic species restricted to a
particular region
endergonic reaction metabolic
reaction requiring energy input
endocrine glands the hormone-
producing glands that release
secretions directly into the body
fluids
endocytosis uptake of fluid or tiny
particles into vacuoles in the
cytoplasm, carried out at the plasma
membrane
endoplasmic reticulum system of
branching membranes in the
cytoplasm of eukaryotic cells,
existing as rough ER (with
ribosomes) or as smooth ER
(without ribosomes)
endosperm the stored food reserves
within the seeds of flowering plants
endoskeleton an internal skeleton
system
endothermic generation of body heat
metabolically
endothelium a single layer of cells
lining blood vessels and other fluid-
filled cavities
enzyme mainly proteins (a very few
are RNA) that function as
biological catalysts
epidemiology the study of the
occurrence, distribution and control
of disease
epidermis outer layer(s) of cells
epiglottis flap of cartilage that closes
of the trachea when food is
swallowed
epiphyte plant living on the surface
of other plants
epithelium sheet of cells bound
strongly together, covering internal
or external surfaces of multicellular
organisms
erythrocyte red blood cell
estimate find an approximate value
for an unknown quantity, based on
the information provided and
scientific knowledge
etiolation the condition of plants
when grown in the dark
eukaryotic (cells) cells with a ‘good’
nucleus (e.g. animal, plant, fungi
and protoctista cells)
evaluate assess the implications and
limitations
evolution cumulative change in the
heritable characteristics of a
population
ex situ not in its original or natural
position or habitat
excretion removal from the body of
the waste products of metabolic
pathways
exergonic reaction metabolic
reaction releasing energy
exocytosis secretion of liquids and
suspensions of very fine particles
across the membrane of eukaryotic
cells
exocrine gland gland whose secretion
is released via a duct
exoskeleton skeleton secreted
external to the epidermis of the
body
exothermic chemical reaction that
releases energy as heat (an
endothermic reaction requires heat
energy)
explain give a clear account including
causes, reasons or mechanisms
expiratory emitting air during
breathing
extensor muscle a muscle that
extends or straightens a limb
F
F1 generation first filial generation –
arise by crossing parents (P), and
when selfed or crossed via sibling
crosses, produce the F2 generation
facilitated diffusion diffusion across a
membrane facilitated by molecules
in the membrane (without the
expenditure of metabolic energy)
fermentation anaerobic breakdown of
glucose, with end-products ethanol
and carbon dioxide or lactic acid
fetus a mammalian embryo when it
becomes recognisable (e.g. the
human embryo from 7 weeks after
fertilisation)
fertilisation the fusion of male and
female gametes to form a zygote
field layer the layer of herbaceous
plants in a forest or wood
filter-feeding feeding on tiny
organisms which are strained from
the surrounding medium
fimbria (singular, fimbrium) thin,
short filaments protruding from
some bacteria, involved in
attachment
flaccid state of a tissue with
insufficient water, as in wilting leaves
flagellum (plural, flagella) a long thin
structure, occurring singly or in
groups on some cells and tissues,
and used to propel unicellular
organisms, and to move liquids past
anchored cells (flagella of
prokaryotes and eukaryotes are of
different internal structure)
flexor muscle a muscle that on
contraction bends a limb (or part of
a limb)
flower develops from the tip of a
shoot, with outer parts (e.g. sepals,
petals) surrounding the male and
female reproductive organs
fluid mosaic model the accepted view
of the structure of the plasma
membrane, comprising a
phospholipid bilayer with proteins
embedded but free to move about
food chain a sequence of organisms
within a habitat in which each is
the food of the next, starting with a
producer, which is photosynthetic
food web interconnected food chains
founder effect genetic differences
that develop between an original
breeding population and a small
isolated interbreeding group of
these organisms
fovea point on a retina of greatest
acuity of vision
free energy part of the potential
chemical energy in molecules that
is available to do useful work when
the molecules are broken
frequency commonness of an
occurrence
fruit forms from the ovary after
fertilisation, as the ovules develop
into seeds
functional group the chemically
active part of a member of a series
of organic molecules
fungus heterotrophic, non-motile,
multicellular (usually) eukaryotic
organism with ‘plant’ body – a
mycelium of hyphae with cell walls
of chitin; the fungi constitute a
separate kingdom

6. G
gall bladder sac beside the liver that
stores bile, present in some
mammals (e.g. humans)
gamete sex cell (e.g. ovum, sperm)
ganglion part of a nervous system,
consisting of nerve cell bodies
gaseous exchange exchange of
respiratory gases (oxygen, carbon
dioxide) between cells/organism
and the environment
gastric relating to the stomach
gene a heritable factor that controls a
specific characteristic
gene mutation change in the
chemical structure (base sequence)
of a gene resulting in change in the
characteristics of an organism or
individual cell
gene pool all the genes (and their
alleles) present in a breeding
population
gene probe an artificially prepared
sequence of DNA made radioactive
with 14C, coding for a particular
amino acid residue sequence
gene therapy various mechanisms by
which corrected copies of genes are
introduced into a patient with a
genetic disease
generator potential localised
depolarisation of a membrane of a
sensory cell
genetic code the order of bases in
DNA (of a chromosome) that
determines the sequence of amino
acids in a protein
genetic counselling genetic advice to
potential parents on the risks of
having children with an inherited
disease
genetic engineering change to the
genetic constitution of individuals
or populations by artificial
selection
genome the genetic complement
(genes) of an organism or of an
individual cell – the whole of the
genetic information of an organism
genotype the genetic constitution of
an organism – the alleles of an
organism
genus a group of similar and closely
related species
germination the resumption of
growth by an embryonic plant in
seed or fruit, at the expense of
stored food
gland cells or tissues adapted for
secretion
global warming the hypothesis that
the world climate is warming due to
rising levels of atmospheric carbon
dioxide, a greenhouse gas
glomerulus network of capillaries
which are surrounded by the renal
capsule
glycocalyx long carbohydrate
molecules attached to membrane
proteins and membrane lipids
glycogen a much-branched polymer
of glucose, the storage carbohydrate
of many animals
glycogenesis the synthesis of glycogen
from glucose (the reverse is
glycogenolysis)
glycolysis the first stage of tissue
respiration in which glucose is
broken down to pyruvic acid,
without use of oxygen
glycoprotein membrane protein with
a glycocalyx attached
glycosidic bond a type of chemical
linkage between monosaccharide
residues in polysaccharides
goblet cell mucus-secreting cell of an
epithelium
Golgi apparatus a stack of flattened
membranes in the cytoplasm, the
site of synthesis of biochemicals
gonad an organ in which gametes are
formed
gonadotrophic hormone follicle-
stimulating hormone (FSH) and
luteinising hormone (LH), secreted
by the anterior pituitary, which
stimulate gonad function
granum (plural, grana) stacked disks
of membranes found within the
chloroplast, containing the
photosynthetic pigments, and the
site of the light-dependent reaction
of photosynthesis
grey matter regions of the brain and
spinal cord consisting largely of
nerve cell bodies
growth more or less irreversible
increase in size and amount of dry
matter
gut the alimentary canal
H
habitat the locality or surroundings in
which an organism normally lives
or the location of a living organism
haemoglobin a conjugated protein,
found in red cells, effective at
carrying oxygen from regions of
high partial pressure (e.g. lungs) to
regions of low partial pressure of
oxygen (e.g. respiring tissues)
half-life the time taken for the ionising
radiation emitted by a radioactive
isotope to fall to half maximum
hallucinogen a drug capable of
causing hallucinations
halophyte a plant adapted to survive
at abnormally high salt levels (e.g.
seashore or salt marsh plant)
haploid (cells) cells having one set of
chromosomes, the basic set
heart rate number of contractions of
the heart per minute
hepatic associated with the liver
herb layer layer of herbaceous plants
(mainly perennials) growing in
woodland
herbaceous non-woody
herbicide pesticide toxic to plants
herbivore an animal that feeds
(holozoically) exclusively on plants
hermaphrodite organism with both
male and female reproductive
systems
heterotroph an organism incapable of
synthesising its own elaborated
nutrients
heterozygous having two different
alleles of a gene
hexose a monosaccharide containing
six carbon atoms (e.g. glucose,
fructose)
hibernation passing the unfavourable
season in a resting state of sleep
histology the study of the structure of
tissues
histone basic proteins (rich in the
amino acids arginine and lysine)
that form the scaffolding of
chromosomes
holozoic ingesting complex food
material and digesting it
homeostasis maintenance of a
constant internal environment
homeotherm organism that maintains
a constant body temperature
B412 Glossary

7. Glossary B413
homologous chromosomes
chromosomes in a diploid cell which
contain the same sequence of genes,
but are derived from different parents
homologous structures similar due to
common ancestry
homozygous having two identical
alleles of a gene
hormone a substance, formed by an
endocrine gland and transported in
the blood all over the body, but
triggering a specific physiological
response in one type of organ or
tissue
host an organism in or on which a
parasite spends all or part of its life
cycle
humus complex organic matter, the
end-product of the breakdown of
the remains of plants and animals,
which covers the mineral particles
of soil
hybrid an individual produced from a
cross between two genetically
unlike parents
hybridoma an artificially produced
hybrid cell culture, used to produce
monoclonal antibodies
hydrocarbon chain a linear
arrangement of carbon atoms
combined together and with
hydrogen atoms, forming a
hydrophobic tail to many large
organic molecules
hydrogen bond a weak bond caused
by electrostatic attraction between
a positively charged part of one
molecule and a negatively charged
part of another
hydrolysis a reaction in which
hydrogen and hydroxide ions from
water are added to a large molecule
causing it to split into smaller
molecules
hydrophilic water loving
hydrophobic water hating
hydrophyte an aquatic plant
hydrosere a plant succession that
originated from open water
hydrostatic pressure mechanical
pressure exerted on or by liquid
(e.g. water) also known as pressure
potential
hyperglycaemia excess glucose in the
blood
hypertonic solution a more
concentrated solution (one with a
less negative water potential) than
the cell solution
hypha the tubular filament ‘plant’
body of a fungus, which in certain
species is divided by cross walls into
either multicellular or unicellular
compartments
hypoglycaemia very low levels of
blood glucose
hypothalamus part of floor of the rear
of the forebrain, a control centre for
the autonomic nervous system, and
source of releasing factors for
pituitary hormones
hypothesis a tentative (and testable)
explanation of an observed
phenomenon or event
hypotonic solution a less
concentrated solution (one with a
more negative water potential) than
the cell solution
I
identify find an answer from a
number of possibilities
immunisation (e.g. inoculation/
vaccination) the injection of a
specific antigen, derived from a
pathogen, to confer immunity
against a disease
immunity resistance to the onset of a
disease after infection by the
causative agent
active immunity immunity due to
the production of antibodies by the
organism itself after the body’s
defence mechanisms have been
stimulated by antigens
passive immunity immunity due to
the acquisition of antibodies from
another organism in which active
immunity has been stimulated,
including via the placenta,
colostrum, or by injection of
antibodies
immunoglobin proteins synthesised
by the B lymphocytes of the
immune system
immunology study of the immune
system
immunosuppressant a substance
causing temporary suppression of
the immune response
implantation embedding of the
blastocyst (developed from the
fertilised ovum) in the uterus wall
impulse see action potential
imprinting process occurring soon
after birth, causing young birds
follow their mother
in situ in the original place (in the
body or organism)
in vitro biological processes occurring
in cell extracts (literally ‘in glass’)
in vivo biological process occurring in
a living organism (literally ‘in life’)
inbreeding when gametes of closely
related individuals fuse leading to
progeny that is homozygous for
some or many alleles
incubation period period between
infection by a causative agent and
the appearance of the symptoms of
a disease
incus tiny, anvil-shaped bone, the
middle ossicle of the middle ear in
mammals
industrial melanism increasing
proportion of a darkened (melanic)
form of an organism, in place of the
light-coloured form, associated with
industrial pollution by soot
infectious disease disease capable of
being transmitted from one
organism to another
inhibitor (enzyme) a substance which
slows or blocks enzyme action (a
competitive inhibitor binds to the
active site; a non-competitive
inhibitor binds to another part of
the enzyme)
inhibitory synapse synapse at which
arrival of an impulse blocks forward
transmissions of impulses in the
post-synaptic membrane
innate behaviour behaviour that does
not need to be learned
innervation nerve supply
inspiratory capacity amount of air
that can be drawn into the lungs
intelligence the ability to learn by
reasoning and to solve problems not
yet experienced
interferon proteins formed by
vertebrate cells in response to virus
infections
intermediates metabolites formed as
components of a metabolic
pathway

8. interphase the period between nuclear
divisions when the nucleus controls
and directs the activity of the cell
interspecific competition competition
between organisms of different species
intestine the gut
intracellular enzymes enzymes
operating inside the cell
intraspecific competition competition
between organisms of the same
species
intron a non-coding nucleotide
sequence of the DNA of
chromosomes, present in eukaryotic
chromosomes
invagination the intucking of a
surface or wall
ion charged particle formed by the
transfer of electron(s) from one
atom to another
ionic bonding strong electrostatic
attraction between oppositely
charged ions
iris circular disc of tissue, in front of
the lens of the eye, containing
circular and radial muscles
irreversible inhibition inhibition by
inhibitors that bind tightly and
permanently to an enzyme,
destroying its catalytic properties
islets of Langerhans groups of
endocrine cells scattered through
the pancreas
isomers chemical compounds of the
same chemical formula but different
structural formulae
isotonic being of the same osmotic
concentration and therefore of the
same water potential
isotopes different forms of an
element, chemically identical but
with slightly different physical
properties, based on differences in
atomic mass (due to different
numbers of neutrons in the nucleus)
J
joule the SI unit of energy
K
keratin a fibrous protein found in
horn, hair, nails, and in the upper
layer of skin
kinesis random movements
maintained by motile organisms
until more favourable conditions
are reached
kinetic energy energy in movement
kingdom the largest and most
inclusive group in taxonomy
Krebs cycle part of tissue respiration
L
label add labels to a diagram
lactation secretion of milk in
mammary glands
leaching washing out of soluble ions
and nutrients by water drainage
through soil
learned behaviour in animals,
behaviour that is consistently
modified as a result of experiences
leucocyte white blood cell
lichens permanent, mutualistic
associations between certain fungi
and algae, forming organisms found
encrusting walls, tree trunks and
rocks
ligament strong fibrous cord or
capsule of slightly elastic fibres,
connecting movable bones
light-independent step part of
photosynthesis occurring in the
stroma of the chloroplasts and using
the products of the light-dependent
step to reduce carbon dioxide to
carbohydrate
light-dependent step part of
photosynthesis occurring in grana of
the chloroplasts, in which water is
split and ATP and NADPH2 are
regenerated
lignin complex chemical impregnating
the cellulose of the walls of xylem
vessels, fibres and tracheids,
imparting great strength and rigidity
lipid diverse group of organic
chemicals essential to living things,
insoluble in water but soluble in
organic solvents such as ether and
alcohol (e.g. lipid of the plasma
membrane)
linkage group the genes carried on
any one chromosome
lipoprotein a complex of lipid and
protein of various types which are
classified according to density (e.g.
LDL, HDL)
list give a sequence of names or other
brief answers with no elaboration
liver lobule polygonal block of liver
cells, a functional unit within the
liver structure
locus the particular position on
homologous chromosomes of a gene
loop of Henle loop of mammalian
kidney tubule, passing from cortex
to medulla and back, important in
the process of concentration of
urine
lumen internal space of a tube (e.g.
gut, artery, etc.) or sac-shaped
structure
lymph fluid derived from plasma of
blood, bathing all tissue spaces and
draining back into the lymphatic
system
lymph node tiny glands in the
lymphatic system, part of the body’s
defences against disease
lymphatic system network of fine
capillaries throughout the body of
vertebrates, which drain lymph and
return it to the blood circulation
lymphocyte type of white blood cell
lysis breakdown, typically of cells
lysosome membrane-bound vesicles,
common in the cytoplasm,
containing digestive enzymes
M
macromolecule very large organic
molecule – rmm 10 000+ (e.g.
protein, nucleic acid or
polysaccharide)
macronutrients ions required in
relatively large amounts by
organisms
Malpighian body glomerulus and
renal capsule of mammalian
nephron
mandibles the lower jaw of
vertebrates; in arthropods paired,
biting mouthparts
matrix ground substance of
connective tissue, and the
innermost part of a mitochondrion
measure find a value for a quantity
mechanoreceptors a sensory receptor
sensitive to mechanical stimulus
meiosis nuclear division with daughter
cells containing half the number of
chromosomes of the parent cell
melanic pigmented
menstrual cycle monthly cycle of
ovulation and menstruation in
human females
meristem plant tissue capable of
giving rise to new cells and tissues
B414 Glossary

9. Glossary B415
mesentery connective tissue holding
body organs (e.g. gut) in position
mesophyll parenchyma cells
containing chloroplasts
mesosome an invagination of the
plasma membrane of a bacterium
metabolic pathway sequence of
enzyme-catalysed biochemical
reactions in cells and tissues
metabolic water water released
within the body by oxidation,
typically of dietary lipids
metabolism integrated network of all
the biochemical reactions of life
metabolite a chemical substance
involved in metabolism
metaphase stage in nuclear division
(mitosis and meiosis) in which
chromosomes become arranged at
the equator of the spindle
microhabitat the environment
immediately surrounding an
organism, particularly applied to
tiny organisms
micronutrient ions required in
relatively small (trace) amounts by
organisms
microtubule tiny, hollow protein tube
in cytoplasm (e.g. a component of
the spindle)
microvillus one of many tiny
infoldings of the plasma membrane,
making up a brush border
middle lamella a layer of pectins
between the walls of adjacent cells
mitochondrion (plural, mitochondria)
organelle in eukaryotic cells, site of
Krebs cycle and the electron-
transport pathway
mitosis nuclear division in which the
daughter nuclei have the same
number of chromosomes as the
parent cell
mitral valve left atrio-ventricular
valve
mode the most frequently occurring
value in a distribution
monoclonal antibody antibody
produced by a single clone of B
lymphocytes; it consists of a
population of identical antibody
molecules
monocotyledon class of angiosperms
having an embryo with a single
cotyledon
monocyte large phagocytic white
blood cell
monohybrid cross a cross (breeding
experiment) involving one pair of
contrasting characters exhibited by
homozygous parents
monosaccharide simple carbohydrate
(all are reducing sugars)
morphology form and structure of an
organism
motile capable of moving about
motor area area of the brain where
muscular activity is coordinated
motor end plate the point of
termination of an axon in a
voluntary muscle fibre
motor neurone nerve cell that carries
impulses away from the central
nervous system to an effector (e.g.
muscle, gland)
mRNA single-stranded ribonucleic
acid formed by the process of
transcription of the genetic code in
the nucleus, that then moves to
ribosomes in the cytoplasm
mucilage mixture of various
polysaccharides that become
slippery when wet
mucosa the inner lining of the gut
mucus a watery solution of
glycoprotein with protective and
lubrication functions
muscle spindle sensory receptor in
muscle, responding to stretch
stimuli
mutagen an agent that causes
mutation
mutant organism with altered genetic
material (abruptly altered by a
mutation)
mutation a change in the amount or
the chemical structure (i.e. base
sequence) of DNA of a
chromosome
mutualism a case of symbiosis in
which both organisms benefit from
the association
mycelium a mass or network of
hyphae
mycology the study of fungi
mycorrhiza a mutualistic association
between plant roots and fungi, with
the mycelium restricted to the
exterior of the root and its cells
(ectotrophic), or involving a closer
association between hyphae and
root cell contents (endotrophic)
myelin sheath an insulating sheath of
axons of nerve fibres, formed by the
wrapping around of Schwann cells
myelinated nerve fibre nerve fibre
insulated by a lipid sheath formed
from membranes of Schwann cells
myofibril contractile protein filament
from which muscle is composed
myogenic originating in heart muscle
cells themselves, as in generation of
the basic heart beat
N
natural classification organisms
grouped by as many common
features as possible, and therefore
likely to reflect evolutionary
relationships
nectary group of cells secreting nectar
(dilute sugar solution) in a flower
nematocyst stinging cell of cnidarians
(coelenterates) (e.g. Hydra)
Neolithic revolution the period of
human development involving the
first establishment of settled
agriculture practices, and including
the breeding and cultivation of crop
plants and herd animals
nephron the functional unit of a
vertebrate kidney
nerve bundle of many nerve fibres
(axons), connecting the central
nervous system with parts of the body
nerve cord in non-vertebrates, a
bundle of nerve fibres and/or nerve
ganglia running along the length of
the body
nervous system organised system of
neurones which generate and
conduct impulses
autonomic nervous system (ANS)
the involuntary nervous system
central nervous system (CNS) in
vertebrates, the brain and spinal cord
parasympathetic nervous system
part of the involuntary nervous
system, antagonistic in effect to the
sympathetic nervous system
peripheral nervous system (PNS)
in vertebrates, neurones that
convey sensory information to the
CNS, and neurones that convey
impulses to muscles and glands
(effector organs)

10. sympathetic nervous system part
of the involuntary nervous system,
antagonistic in effect to the
parasympathetic nervous system
neurone nerve cell
neurotransmitter substance chemical
released at the pre-synaptic
membrane of an axon, on arrival of
an action potential, which transmits
the action potential across the
synapse
neutrophil a type of white blood cell
niche both the habitat an organism
occupies and the mode of nutrition
employed
node of Ranvier junction in the
myelin sheaths around a myelinated
nerve fibre
noradrenaline neurotransmitter
substance in the sympathetic
nervous system
nuclear division first step in the
division of a cell, when the
contents of the nucleus are
subdivided by mitosis or meiosis
nuclear membrane double membrane
surrounding the eukaryotic nucleus
nuclear pores organised gaps in the
nuclear membrane, exit points for
mRNA
nucleic acid polynucleotide chain of
one of two types, deoxyribonucleic
acid (DNA) or ribonucleic acid
(RNA)
nucleus largest organelle of
eukaryotic cells; controls and directs
the activity of the cell
nucleolus compact region of nucleus
where RNA is synthesised
nucleoside organic base (adenine,
guanine, cytosine, thymine)
combined with a pentose sugar
(ribose or deoxyribose)
nucleotide phosphate ester of a
nucleoside – an organic base
combined with pentose sugar and
phosphate (Pi)
nutrient a chemical substance found
in foods that is used in the human
body – any substance used or
required by an organism as food
nutrition the process by which an
organism acquires the matter and
energy it requires from its
environment
O
obesity condition of being seriously
over-weight (BMI of 30+)
oestrous cycle reproductive cycle in
female mammal in the absence of
pregnancy
oestrous period of fertility
(immediately after ovulation)
during the oestrous cycle
olfactory relating to the sense of smell
omnivore an animal that eats both
plant and animal food
oncogene a cancer-initiating gene
oocyte a female sex cell in the
process of a meiotic division to
become an ovum
oogamy union of unlike gametes (e.g.
large ovum and tiny sperm)
opsonin type of antibody that attacks
bacteria and viruses, facilitating
their ingestion by phagocytic cells
order a group of related families
organ a part of an organism,
consisting of a collection of tissues,
having a definite form and
structure, and performing one or
more specialised functions
organelle a unit of cell substructure
organic compounds of carbon (except
carbon dioxide and carbonates)
organism a living thing
osmoreceptor sense cells or organ
stimulated by changes in water
potential
osmoregulation control of the water
balance of the blood, tissue or
cytoplasm of a living organism
osmosis diffusion of free water
molecules from a region where they
are more concentrated (low solute
concentration) to a region where
they are less concentrated (high
solute concentration) across a
partially permeable membrane
outline give a brief account or
summary
ovarian cycle the monthly changes
that occur to ovarian follicles
leading to ovulation and the
formation of a corpus luteum
ovary female reproductive organ in
which the female gametes are
formed
ovarian follicle spherical structures
found in the mammalian ovary,
containing a developing ovum with
liquid surrounded by numerous
follicle cells, and from which a
secondary oocyte is released at
ovulation
ovum (plural, ova) a female gamete
ovulation shedding of ova from the
ovary
ovule in the flowering plant flower,
the structure in an ovary which, after
fertilisation, grows into the seed
oxygen dissociation curve a graph of
% saturation (with oxygen) of
haemoglobin against concentration
of available oxygen
oxyntic cells cells in the gastric
glands secreting hydrochloric acid
P
pacemaker structure that is the origin
of the myogenic heart beat, known
as the sino-atrial node
Pacinian corpuscles sensory receptors
in joints
pancreas an exocrine gland discharging
pancreatic juice into the duodenum,
combined with endocrine glands
(islets of Langerhans)
parasite an organism that lives on or
in another organism (its host) for
most of its life cycle, deriving
nutrients from its host
parenchyma living cells, forming the
greater part of cortex and pith in
primary plant growth
pathogen an organism or virus that
causes a disease
partial pressure the pressure exerted
by each component of a gas
mixture, proportional to how much
of the gas is present in the mixture;
the partial pressure of oxygen in air
is represented by the symbol pO2
and is expressed in kilopascals (kPa)
pentadactyl having all four limbs
(typically) terminating in five digits
pentose a 5-carbon monosaccharide
sugar
peptide a chain of up to 20 amino
acid residues, linked by peptide
linkages
peptide linkage a covalent bonding of
the α amino group of one amino
acid to the carboxyl group of
another (with the loss of a molecule
of water)
B416 Glossary

11. Glossary B417
perception the mental interpretation
of sense data (i.e. occurring in the
brain)
pericardium a tough membrane
surrounding and containing the heart
peristalsis wave of muscular
contractions passing down the gut
wall
pesticide a chemical that is used to
kill pests
petal modified leaf, often brightly
coloured, found in flowers
phagocytic cells cells that ingest
bacteria etc. (e.g. certain
leucocytes, Amoeba)
phenotype the characteristics or
appearance (structural, biochemical,
etc.) of an organism
pheromone volatile chemical signal
released into the air
phloem tissue that conducts
elaborated food in plant stems
phosphate (Pi) phosphate ions, as
involved in metabolism
phospholipid formed from a
triacylglycerol in which one of the
fatty acid groups is replaced by an
ionised phosphate group
photoautotroph an organism that
uses light energy to generate ATP
and to produce organic compounds
from inorganic substances
photoheterotroph an organism that
uses light energy to generate ATP
and obtains organic compounds
from other organisms
photomorphogenesis effects on plant
growth of light
photoperiodism day-length control of
flowering in plants
photosynthesis the production of
sugar from carbon dioxide and
water, occurring in chloroplasts and
using light energy, and producing
oxygen as a waste product
photophosphorylation the formation
of ATP, using light energy (in the
light-dependent step of
photosynthesis in the grana)
phototropism a tropic response of
plants to light
phylogenetic classification a
classification based on evolutionary
relationships (rather than on
appearances)
phylum a group of organisms
constructed on a similar general
plan, usually thought to be
evolutionarily related
physiology the study of the
functioning of organisms
phytoplankton photosynthetic
plankton, including unicellular
algae and cyanobacteria
pinocytosis uptake of a droplet of
liquid into a cell involving
invagination of the plasma membrane
pituitary gland the master endocrine
gland, attached to the underside of
the brain
placenta maternal and fetal tissue in
the wall of the uterus, site of all
exchanges of metabolites and waste
products between fetal and
maternal blood systems
plant growth substance substances
produced by plants in relatively
small amounts, that interact to
control growth and development
plasma the liquid part of blood
plasma membrane the membrane of
lipid and protein that forms the
surface of cells (constructed as a
fluid mosaic membrane)
plasmid small circular DNA that is
independent of the chromosome in
bacteria (R plasmids contain genes
for resistance to antibiotics)
plasmolysis withdrawal of water from
a plant cell by osmosis (incipient
plasmolysis is established when
about 50% of cells show some
shrinkage of cytoplasm away from
the walls)
plankton very small, aquatic (marine
or fresh water) plants and animals,
many of them unicellular, that live
at or near the water’s surface
plastid an organelle containing
pigments (e.g. chloroplast)
platelets tiny cell fragments that lack
a nucleus, found in the blood and
involved in the blood clotting
mechanism
pleural membrane lines lungs and
thorax cavity and contains the
pleural fluid
polarise the setting up of an electrical
potential difference across a
membrane
polarised light light in which rays
vibrate in one plane only
pollen microspore produced in
anthers (and male cones),
containing male gamete(s)
pollen tube grows out of a pollen
grain attached to a stigma, and
down through the style tissue to the
embryo sac
polygenic inheritance inheritance of
phenotypic characters (such as
height, eye colour in humans) that
are determined by the collective
effects of several different genes
polynucleotide a long, unbranched
chain of nucleotides, as found in
DNA and RNA
polymer large organic molecules
made up of repeating subunits
(monomers)
polypeptide a chain of amino acid
residues linked by peptide linkages
polyploidy having more than two sets
of chromosomes per cell
polysaccharides very high molecular
mass carbohydrates, formed by
condensation of vast numbers of
monosaccharide units, with the
removal of water
polysome an aggregation of ribosomes
along a molecule of mRNA strand
population a group of organisms of
the same species which live in the
same area (habitat) at the same time
portal vein vein beginning and
ending in a capillary network
(rather than at the heart)
post-synaptic neurone neurone
‘downstream’ of a synapse
potential difference separation of
electrical charge within or across a
structure (e.g. a membrane)
potential energy stored energy
predator an organism that catches
and kills other animals to eat
predict give an expected result
pre-synaptic membrane membrane of
the tip of an axon at the point of
the synapse
pre-synaptic neurone neurone
‘upstream’ of a synapse
prey–predator relationship the inter-
relationship of population sizes due
to predation of one species (the
predator) on another (the prey)

12. proboscis a projection from the head,
used for feeding
producer an autotrophic organism
productivity the amount of biomass
fixed by producers
(photosynthetically)
gross productivity total amount of
organic matter produced
net productivity the organic
matter of organisms less the amount
needed to fuel respiration
prokaryote tiny unicellular organism
without a true nucleus; they have a
ring of RNA or DNA as a
chromosome (e.g. bacteria and
cyanobacteria)
prophase first stage in nuclear
division, mitotic or meiotic
proprioceptor an internal sensory
receptor
prosthetic group a non-protein
substance, bound to a protein as
part of an enzyme, often forming
part of the active site, and able to
bind to other proteins
protein a long sequence of amino
acid residues combined together
(primary structure), and taking up a
particular shape (secondary and
tertiary structure)
Protoctista kingdom of the
eukaryotes consisting of single-
celled organisms and multicellular
organisms related to them (e.g.
protozoa and algae)
protoplast the living contents of a
plant cell, contained by the cell wall
protozoan a single-celled animal-like
organism, belonging to a sub-
kingdom, the Protozoa, of the
kingdom Protoctista
pseudopodium a temporary extension
of the body of an amoeboid cell, by
which movement or feeding may
occur
pulmonary circulation the circulation
to the lungs in vertebrates having a
double circulation
pulmonary ventilation rate breathing
rate
pulse a wave of increased pressure in
the arterial circulation, generated
by the heart beat
pumps proteins in plasma membranes
that use energy directly to carry
substances across (primary pump) or
work indirectly from metabolic
energy (secondary pump)
pupil central aperture in the eye
through which light enters
pure breeding homozygous, at least
for the gene(s) specified
Purkinje fibres fibres of the bundle of
His that conduct impulses between
the atria and ventricles of the heart
pyloric sphincter circular muscle at
the opening of the stomach to the
duodenum
pyruvic acid a 3-carbon organic acid,
CH3.CO.COOH; product of
glycolysis
Q
quadrat a sampling area enclosed
within a frame
R
radical a short-lived, intermediate
product of a reaction, formed when
a covalent bond breaks, with one of
the two bonding electrons going to
each atom
radioactive dating using the
proportions of different isotopes in
fossilised biological material to
estimate when the original
organism was alive
reaction centres protein–pigment
complexes in the grana of
chloroplasts, sites of the
photochemical reactions of
photosynthesis
receptor a sense organ
recessive allele an allele that has an
effect on the phenotype only when
present in the homozygous state
reciprocal cross a cross between the
same pair of genotypes in which the
sources of the gametes (male and
female) are reversed
recombinant a chromosome (or cell
or organism) in which the genetic
information has been rearranged
recombinant DNA DNA which has
been artificial changed, involving
joining together genes from
different sources, typically from
different species
recycling of nutrients the process by
which materials from dead organisms
are broken down and made
available for re-use in the biosphere
Red Data Book an internationally
produced record of actions for
endangered species
redox reaction reaction in which
reduction and oxidation happen
simultaneously
reductive division meiosis, in which
the chromosome number of a
diploid cell is halved
reflex a rapid unconscious response
reflex action a response automatically
elicited by a stimulus
reflex arc a functional unit in the
nervous system, consisting of
sensory receptor, sensory neurone,
(possibly relay neurones), motor
neurone and effector (e.g. muscle or
gland)
refractory period the period after
excitation of a neurone, when a
repetition of the stimulus fails to
induce the same response, divided
into periods known as absolute and
relative
relative atomic mass the ratio of the
mass of an atom of an element to
the mass of a carbon atom
renal capsule the cup-shaped closed
end of a nephron which, with the
glomerulus, constitutes a
Malpighian body
renewable energy energy that comes
from exploiting wave power, wind
power, tidal power, solar energy,
hydroelectric power or biological
sources such as biomass
replication duplication of DNA by
making a copy of an existing
molecule
semi-conservative replication each
strand of an existing DNA double
helix acts as the template for the
synthesis of a new strand
reproduction formation of new
individual by sexual or asexual means
residual volume volume of air
remaining in the lungs after
maximum expiration
respiration the cellular process by
which sugars and other substances
are broken down to release chemical
energy for other cellular processes
respiratory centre region of the
medulla of the brain concerned with
the involuntary control of breathing
B418 Glossary

13. Glossary B419
respiratory pigment substance such as
haemoglobin, which associates with
oxygen
respiratory quotient ratio of the
volume of carbon dioxide produced
to the oxygen used in respiration
respiratory surface a surface adapted
for gaseous exchange
respirometer apparatus for the
measurement of respiratory gaseous
exchange
response the outcome when a
stimulus is detected by a receptor
resting potential the potential
difference across the membrane of a
neurone when it is not being
stimulated (repolarised)
restriction enzymes enzymes, also
known as endonucleases, that cut
lengths of nucleic acid at specific
sequences of bases
retina the light-sensitive layer at the
back of the eye
retroviruses viruses which, on arrival in
a host cell, have their own RNA
copied into DNA which then
attaches to the host DNA for a period
ribosome non-membranous organelle,
site of protein synthesis
ribonucleic acid (RNA) a form of
nucleic acid containing the pentose
sugar ribose, found in nucleus and
cytoplasm of eukaryotic cells (and
commonly the only nucleic acid of
prokaryotes), and containing the
organic bases adenine, guanine,
uracil and cytosine
rod cell one of two types of light-
sensitive cell in the retina,
responsible for non-colour vision
roughage indigestible matter (such as
cellulose fibres) in our diet
ribulose bisphosphate the 5-carbon
acceptor molecule for carbon
dioxide, in the light-independent
step of photosynthesis
S
saliva secretion produced by salivary
glands
saltatory conduction impulse
conduction ‘in jumps’, between
nodes of Ranvier
saprotroph organism that feeds on
dead organic matter (saprotrophic
nutrition)
sarcolemma membranous sheath
around a muscle fibre
sarcomere a unit of a skeletal
(voluntary) muscle fibre, between
two Z-discs
sarcoplasm cytoplasm around the
myofibril of a muscle fibre
sarcoplasmic reticulum network of
membranes around the myofibrils of
a muscle fibre
saturated fat fat with a fully
hydrogenated carbon backbone (i.e.
no double bonds present)
Schwann cell cell which forms the
sheath around nerve fibres
sclera the opaque, fibrous coat of the
eyeball
secondary sexual characteristic
sexual characteristic that develops
under the influence of sex hormones
(androgens and oestrogens)
secondary succession a plant
succession on soil already formed,
from which the community had
been abruptly removed
secretion material produced and
released from glandular cells
sedentary organism living attached to
the substratum (e.g. rock or other
surface)
seed formed from a fertilised ovule,
containing an embryonic plant and
food store
segmentation body plan built on a
repeating series of similar segments
(e.g. as in annelids)
selection differential survivability or
reproductive potential of different
organisms of a breeding population
self-pollination transfer of pollen from
the anther to the stigma of the same
plant (normally the same flower)
selfing self-pollination or self-
fertilisation
semilunar valve half-moon shaped
valves, preventing backflow in a
tube (e.g. a vein)
seminiferous tubule elongated tubes
in the testes, the site of sperm
production
sense organ an organ of cells
sensitive to external stimuli
sensory area an area of the cerebral
cortex of the brain receiving impulses
from the sense organs of the body
sensory neurone nerve cell carrying
impulses from a sense organ or
receptor to the central nervous
system
sensory receptor a cell specialised to
respond to stimulation by the
production of an action potential
(impulse)
sepal the protective outermost parts
of a flower, usually green
seral stage / sere stages in a seral
succession, the whole succession
being known as a sere
sex chromosome a chromosome
which determines sex rather than
other body (soma) characteristics
sex linkage genes carried on only one
of the sex chromosomes and which
therefore show a different pattern of
inheritance in crosses where the
male carries the gene from those
where the female carries the gene
sexual reproduction involves the
production and fusion of gametes
show give the steps in a calculation
or derivation
shrub layer the low-level (below
trees) woody perennials growing in
a forest or wood, normally most
numerous in clearings (e.g. where a
full-grown tree has died)
sibling offspring of the same parent
sieve tube a phloem element,
accompanied by a companion cell,
and having perforated end walls
known as sieve plates
simple sugar monosaccharide sugar
such as a triose sugar (3C), pentose
sugar (5C), or hexose sugar (6C)
single access key contrasting or
mutually exclusive characteristics
are used to divide the group of
organisms into progressively smaller
groupings until individual organisms
(species) can be identified
sino-atrial node cells in the wall of
the right atrium in which the heart
beat is initiated, also known as the
pacemaker
sinus a cavity or space
sketch represent by means of a graph
showing a line plus labelled but
unscaled axes and with important
features (e.g. intercepts) clearly
indicated

14. solar energy electromagnetic
radiation derived from the fusion of
hydrogen atoms of the Sun,
reaching Earth from space
solve obtain an answer using
algebraic and/or numerical methods
somatic cell (soma) body cell – not a
cell producing gametes (sex cell)
specialisation adaptation for a
particular mode of life or function
speciation the evolution of new
species
species a group of individuals of
common ancestry that closely
resemble each other and that are
normally capable of interbreeding
to produce fertile offspring
sperms motile male gametes of
animals
spermatogonia male germ cells (stem
cells) which make up the inner
layer of the lining of the
seminiferous tubules, and give rise
to spermatocytes
spermocyte cell formed in
seminiferous tubules of testes;
develops into sperm
spindle structure formed from
microtubules, associated with the
movements of chromosomes in
mitosis and meiosis
spiracle hole in the side of an insect
(thorax and abdomen) by which
the tracheal respiratory system
connects with the atmosphere
spiral vessel protoxylem vessel with
spirally arranged lignin thickening
in lateral walls
spirometer apparatus for
measurements of lung capacity and
breathing rates
spore a small, usually unicellular
reproductive structure from which a
new organism arises
standing crop the biomass of a
particular area under study
stamen male reproductive organ of
the flower, consisting of filament
and anther, containing pollen sacs
where pollen is formed and released
state give a specific name, value or
other brief answer (no supporting
argument or calculation is
necessary)
steroid organic molecule formed from
a complex ring of carbon atoms, of
which cholesterol is a typical
example
stigma part of the carpel receptive to
pollen
stimulus a change in the
environment (internal or external)
that is detected by a receptor and
leads to a response
stoma (plural, stomata) pore in the
epidermis of a leaf, surrounded by
two guard cells
stretch receptor sensory receptor in
muscles
stroke volume volume of blood
pumped out by the heart per minute
stroma the membranous matrix of the
chloroplast, site of the light-
independent reaction in
photosynthesis
style found in the female part of the
flower (carpel), linking stigma to
ovary
subthreshold stimulus a stimulus not
strong enough to trigger an action
potential
substrate a molecule that is the
starting point for a biochemical
reaction and that forms a complex
with a specific enzyme
succession the sequences of different
communities developing in a given
habitat over a period of time
sugars compounds of a general
formula Cx(H2O)y, where x is
approximately equal to y, and
containing an aldehyde or a ketone
group
suggest propose a hypothesis or other
possible answer
summation combined effect of many
nerve impulses
spatial many impulses arriving from
different axons
temporal many impulses arriving
via a single axon
suspensory ligament attaches lens to
ciliary body in the vertebrate eye
symbiosis literally ‘living together’;
covering parasitism, commensalism
and mutualism
symplast the pathway (e.g. of water)
through the living contents of cells
synapse the connection between two
nerve cells; functionally a tiny gap,
the synaptic cleft, traversed by
transmitter substances
synaptic knob the terminal swelling
of a pre-synaptic neurone
synergism acting together and
producing a larger effect than when
acting separately
synovial fluid secreted by the
synovial membrane at joints, having
lubricating role
systematics the study of the diversity
of living things
systemic circulation the blood
circulation to the body (not the
pulmonary circulation)
systemic pesticide pesticide that is
absorbed and carried throughout
the body
systole contraction phases in the
cardiac cycle
T
target organ organ on which a
hormone acts (although broadcast
to all organs)
taste bud sense organ found chiefly
on the upper surface of the tongue
taxis response by a motile organism
(or gamete) where the direction of
the response is determined by the
direction of the stimulus
taxon a classificatory grouping
taxonomy the science of classification
telophase a phase in nuclear division,
when the daughter nuclei form
template (DNA) the DNA of the
chromosome, copied to make
mRNA
tendon fibrous connective tissue
connecting a muscle to bone
terminal bud bud at the apex of the
stem
test cross testing a suspected
heterozygote by crossing it with a
known homozygous recessive
testa seed coat
testis male reproductive gland,
producing sperms
thermogenesis generation of heat by
metabolism
testosterone a steroid hormone, the
main sex hormone of male mammals
thorax in mammals, the upper part of
the body separated from the
abdomen; in insects, the region
between head and abdomen
B420 Glossary

15. Glossary B421
threshold of simulation the level of
stimulation required to trigger an
action potential (impulse)
thrombosis blood clot formation,
leading to blockage of a blood vessel
thylakoid membrane system of
chloroplast
thyroid gland an endocrine gland
found in the neck of vertebrates,
site of production of thyroxine and
other hormones influencing the rate
of metabolism
tidal volume volume of air normally
exchanged in breathing
tight junction point where plasma
membranes of adjacent cells are
sealed together
tissue collection of cells of similar
structure and function
tissue fluid the liquid bathing cells,
formed from blood minus cells and
plasma proteins
tissue respiration biochemical steps
by which energy is released from
sugars
tonoplast membrane around the plant
cell vacuole
total lung capacity volume of air in
the lungs after maximum inhalation
toxic poisonous
toxin poison
toxoid inactivated poison
trachea windpipe
tracheal system system of tubes by
which air is passed to tissues in
insects
tracheole branch of the trachea
trait a tendency or characteristic
transcription when the DNA
sequence of bases is converted into
mRNA
transect arbitrary line through a
habitat, selected to sample the
community
transfer RNA (tRNA) short lengths
of specific RNA that combine with
specific amino acids prior to protein
synthesis
translation the information of
mRNA is decoded into protein
(amino acid sequence)
translocation transport of elaborated
food via the phloem
transmitter substances substances
released into the synaptic cleft on
arrival of an impulse at the pre-
synaptic membrane to conduct the
signal across the synapse
transpiration loss of water vapour
from the aerial parts of plants
(leaves and stem)
tricarboxylic acid (TCA) cycle the
stage in tissue respiration in which
pyruvate is broken down to carbon
dioxide, and hydrogen is removed
for subsequent oxidation
tricuspid valve right atrio-ventricular
valve
triglyceride fatty acid ester of the 3-
carbon alcohol, glycerol – forms
into globules because of its
hydrophobic properties
triose a 3-carbon monosaccharide
tripeptide a peptide of three amino
acid residues
trophic level a level in a food chain
defined by the method of obtaining
food and in which all organisms are
the same number of energy
transfers away from the original
source of the energy
(photosynthesis)
tropism a growth response of plants in
which the direction of growth is
determined by the direction of the
stimulus
tumour abnormal proliferation of
cells, either benign (if self-limiting)
or malignant (if invasive)
turgid having high internal pressure
U
ultrafiltration occurs through the
tiny pores in the capillaries of the
glomerulus
ultrastructure fine structure of cells,
determined by electron microscopy
unisexual of one or other sex
unsaturated fat lipid with double
bond(s) in the hydrocarbon chain
urea NH2CONH2, formed from
amino groups deaminated from
excess amino acid
ureter tube from kidney to bladder
urethra tube from bladder to exterior
uterine cycle cycle of changes to the
wall of the uterus (approximately 28
days)
uric acid an insoluble purine, formed
from the breakdown of nucleic acids
and proteins
urine an excretory fluid produced by
the kidneys, consisting largely of a
dilute solution of urea
uterus the organ in which the
embryo develops in female
mammals
V
VO2 the amount of oxygen being
used in the body (cm3 kg−1min−1);
with increasingly vigorous exercise,
VO2 will increase, initially
VO2max the maximal oxygen uptake
by the body (cm3 kg−1min−1) – even
if the maximum physical effort is
maintained, a situation is reached
where further increase is impossible
vaccination conferring immunity from
a disease by injecting an antigen (of
attenuated microorganisms or
inactivated component) so that the
body acquires antibodies prior to
potential infection
vascular bundle strands of xylem and
phloem (often with fibres)
separated by cambium; the site of
water and elaborated food
movements up and down the stem
vacuole fluid-filled space in the
cytoplasm, especially large and
permanent in plant cells
vagus nerve 10th cranial nerve;
supplies many internal organs,
including the heart
variety a taxonomic group below the
species level
vasa recta capillary loop supplying
the loop of Henle
vascular tissue xylem and phloem of
plants
vasoconstriction constriction of
blood supply to capillaries (of skin)
vasodilation dilation of blood supply
to capillaries (of skin)
vector an organism that transmits a
disease-causing organism, or a
device for transferring genes during
genetic engineering
venous return volume of blood
returning to the heart via the veins
per minute
vein vessel that returns blood to the
heart
ventilation rate number of
inhalations or exhalations per
minute

16. ventral the underside
ventricle chamber, either of the
centre of the brain, or of the heart
venule branch of a vein
vertebrate animal with a vertebral
column
vesicle membrane-bound sac
vestibular apparatus the semicircular
canals of the inner ear, concerned
with balance
vestibular canal upper compartment
of the cochlea
vestigial small, imperfectly developed
structure
virus minute, intracellular parasite,
formed of protein and nucleic acid
vital capacity the total possible
change in lung volume – the
maximum volume of air that can be
exhaled after a maximum
inhalation
vitalism theory early idea that
organic compounds could only be
produced in living cells
vitreous humour clear jelly of inner
eye
W
water potential the tendency of water
molecules to move
water table level of ground water in
the Earth
wax complex form of lipid
weathering breakdown of rock
white matter nerve fibres wrapped in
their myelin sheaths
X
xeromorphic modified to withstand
drought
xerophyte plant showing
modifications to withstand drought
xerosere succession of plants starting
from dry terrain
xylem water-conducting vessels of
plants
Y
yolk food stores of egg cells, rich in
proteins and lipids
yolk sac membranous sac with
numerous blood vessels, developed
by vertebrate embryos around the
yolk (e.g. in birds and reptiles) or as
a component of the placenta (in
mammals)
Z
zonation naturally occurring
distribution of organisms in zones
zygote product of the fusion of
gametes
zymogenic cells cells of gastric
glands, secreting pepsinogen
B422 Glossary